1. Field of the Invention
This invention pertains generally to human spinal implant systems and methods, and more particularly to implant systems and methods for stabilizing the facet joints of adjacent vertebrae.
2. Description of Related Art
Chronic back problems cause pain and disability for a large segment of the population. In many cases, the chronic back problems are caused by intervertebral disc degeneration and loss of stability of the intervertebral joint. Over time, the process of aging produces progressive dehydration and loss of height of the intervertebral discs. The shortening of the spinal articular segment produces increased mechanical wear of the facet joints. With the increasing pressure across the facet joints, the articular cartilage on each surface of the joints undergoes abrasive thinning, producing pain. Indications for fusion of the spine include trauma, tumor, infection, and; most commonly, the consequences of degenerative change: progressive deformity, and instability producing compression of the neural elements and intractable pain. In the majority of current spinal fusion cases, successful stabilization of the spinal motion segment is achieved through posterior-lateral lumbar fusion, the biomechanical key to which is successful immobilization of the facet joints.
Currently there exists no direct, efficient procedure to achieve consistent fusion or arthrodesis of the posterior-lateral facet joints of the lumbar spine. The vast majority of spine instrumentation systems bypass the facet joint, spanning the posterior vertebral articulations, immobilizing the facet joint indirectly through transpedicular linkage of adjacent vertebrae. There are multiple clinical indications for the performance of a posterior-lateral lumbar fusion and well over 100,000 are performed each year in the United States alone. The ultimate measure of success of spinal fusion is the elimination of motion of the vertebral motion segment. The ultimate measure of success of posteriolateral spinal fusion surgery depends upon solid fusion of the facet joints at each level. For many years the success rate for lumbar fusion surgery has remained unchanged at approximately 85%. The ideal procedure would achieve a 100% predictable spinal fusion using a safe, minimally invasive method to mechanically lock the motion segment in order to allow bone formation and fusion across the facet joints with minimal tissue disruption.
The classic procedures for posterior-lateral fusion of the lumbar spine require long posterior midline incisions, are frequently associated with massive blood loss occurring during prolonged operations, produce widespread paraspinous muscle damage resulting from the extensive retraction forces necessary for visualization, and yield unacceptably high clinical failure rates. The cost of failed spine surgery is enormous. Persistent pain, lost productivity and the disappointingly low success rate of revision surgery underscore the need for more effective primary lumbar spine fusion surgery.
Spinal fusion techniques have evolved significantly since the first successful fusions were reported by Hibbs (Hibbs R A: An operation for progressive spinal deformities. N Y Med J 93:1013-1016) and Albee in 1911 (Albee F H: Transplantation of a portion of the tibia into the spine for Pott's disease. JAMA 57:885-886.) The importance of the facet joint in the success of posterior spine fusion surgery was stressed by McBride in 1948 (McBride E D: A mortised transfacet bone block for lumbosacral fusion. J Bone Joint Surg [AM] 31:385-393.). McBride described a fusion technique to eliminate intervertebral motion at the facet articulation by excavating a rectangular cavity, or mortise, and impacting a dovetailed graft into this area while using intraspinous distraction to assist with graft placement.
The first descriptions of the use of metallic trans-articular implants to obtain internal fixation of the spine as an adjunct to lumbar fusion was reported in the 1940's by Toumey (Toumey J W (1943): Internal fixation in fusion of the lumbosacral joints. Lahey Clin Bull 3:188-191.) and King (King D (1944): Internal fixation for lumbosacral fusion. Am J Surg 66:357-361). These techniques for stabilization of a lumbosacral fusion relied on passing a short bone screw from medial to lateral across the facet joints of the level to be fused bilaterally. Unacceptably high non-union rates associated with these techniques led investigators to develop other methods of spinal instrumentation which spanned or did not rely solely on fixation of the facet joints.
Interest in the facet joint as a key to successful posterior fusion was renewed by Magerl who introduced another variation of facet fixation in which a long slender bone screw was placed from the base of the spinous process on one side, between the two tables of the lamina on the opposite side and then across the facet joint into the base of the transverse process (Magerl F P (1984): Stabilization of the lower thoracic and lumbar spine with external skeletal fixation. Clin Orthop 189:125). Magerl's technique improved the success rate of posterior spine fusions through the use of a long implant fixed to the bone at multiple points which solved the major problem of loosening due to failure of the bone-implant interface. Translaminar facet screw fixation has not been widely adopted since it is much more technically demanding than other current fixation systems, requires multiple incisions, places the dura at risk and depends upon an intact lamina.
Current advances in spine surgical techniques involve the development of minimally invasive, tissue sparing approaches performed through tiny incisions under television image intensifier fluoroscopic guidance.
For these reasons, it would be desirable to provide a new system to provide definitive fixation of the facet joints of the lumbar spine applied through a minimally invasive approach under fluoroscopic guidance.
It would further be desirable to provide a vertebral anatomy training device for demonstrating the precision fluoroscopic beam positioning critical for minimally invasive spine surgery.
It would further be desirable to provide a training aid useful in the initial and follow-up training to aid physicians and their surgical teams in visualizing important aspects of image guided spine surgery procedures.
At least some of these objectives will be met with the invention described herein.